Methods for surface treating or conditioning products fabricated from titanium and titanium alloys are well-known. Broadly, such known methods have included both mechanical and chemical treating or conditioning means. For example, both heavy grinding and lathe turning have been and are used for conditioning the surfaces of heavy products of titanium such as billets and slabs. For the surface conditioning of lighter products of titanium such as sheet, strip, bar and wire products and the like, both mechanical and chemical conditioning means are employed including grit blasting, grinding, caustic descaling and acid pickling.
By far, the most commonly employed means for treating or conditioning the surfaces of products fabricated from titanium is that of acid pickling. The pickling solutions most often employed are mixed solutions of nitric and hydrofluoric acids. Generally, the concentrations of the acid components in such solutions will range from about 15 to 30 weight percent for the nitric acid component and from about 2 to about 4 weight percent for the hydrofluoric acid component in such solutions. According to Kirk-Othmer, Encyclopedia of Chemical Technology, 2ed, Vol 20, page 359 (1969) the higher nitric acid concentrations are employed to minimize hydrogen absorption while the hydrofluoric acid concentration will determine the pickling rate.
Although the above described acid pickling solutions work well for their intended use, i.e., the removal of surface oxide scales formed during the working and annealing processes used in the manufacture of fabricated products of most commercial grades of titanium, their use in the treatment or conditioning of products fabricated from certain titanium base alloy compositions has been found to be detrimental. For example, the use of a pickling solution comprised of nitric and hydrofluoric acids to treat or condition the surfaces of structures fabricated from titanium based alloys containing iron and copper as alloying constituents tends to degrade certain of the desirable inherent properties of these alloys. Particularly, it has been found that when structures fabricated from titanium alloys containing both iron and copper are treated or conditioned with a nitric acid/hydrofluoric acid pickling solution the critical properties of positive open circuit potential and a substantially reduced rate of corrosion inherent in these alloys is significantly degraded, if not lost completely.